1. Field of the Invention
This invention relates generally to apparatus for and the method of bonding and plating work pieces by the use of foil exploded by the application of high intensity electrical current flow, and, more particularly, to improvements therein for producing bonds of high uniformity and strength.
2. Description of the Prior Art
While exploding wire phenomena have been known for a long time, they have been studied intensively only for the past 20 years or so. The usual apparatus used consists of an energy storage device, such as a large capacitor, coupled electrically, through a switch, to a thin metal foil or a wire of small cross section. When the switch is closed, the stored energy is dumped into the wire or foil, causing the current therethrough to rapidly build up, resulting in melting and then vaporization of the wire or foil. The fast change of state of the wire or foil usually causes a shock wave to be formed in the ambient atmosphere surrounding the wire or foil, which is heard as a "pop" or "crack." Thus, the term "exploding wire" aptly describes this procedure.
Various uses for exploding wires or films have been developed. The common fuse is perhaps the best known application. Vaporized metal in a wire explosion has been used to make thin platings inside cylinders, and thin exploding foils have been used to bond quartz rods together. In the latter application, the foil is sandwiched between the work pieces to be bonded and the foil connected to the energy storage circuit. During the explosion, the work pieces ideally trap the vaporized metal between them, and when the hot metal cools, a solid metal bond is formed between the work pieces.
Despite the attractive features of explosive foil bonding, which include rapid (.about.2-5.mu.sec) bond times and simple and inexpensive circuitry and materials, several problems are associated with the use of this method. First, and most importantly, is the formation of striations in the exploded film, which lead to uneven bond areas and an unnecessary waste of film material. These striations, sometimes called "streamers," appear as small sections of the exploded foil, each of which has a long dimension perpendicular to the length of the foil, which is also the direction of current flow. Striations have been observed in laboratory studies through the use of fast X-ray pictures of an exploding film, pictures of the final shapes of exploded films deposited on glass substrates and by analyzing shapes of slag formation when an explosion occurs in a container filled with sand. While the exact cause of striations is not clear, it is believed that their formation is focused in the area of "hot spots" in the foil, which result from uneven current distribution and thus nonuniform heating of the foil during an explosion.
Another problem associated with exploding foils used for bonding and plating is the damage done to the work piece by heat and blast effects. During the explosion temperatures and pressures in excess of 10,000.degree. K and 10.sup.12 dynes/.sub.cm 2, respectively, can easily be reached, although for only very short periods of time. Obviously, it is desirable to reduce these pressures and temperatures as much as possible, in order to prevent unnecessary damage and also to reduce the energy consumed in an explosion. However, sufficient energy must of course be used to produce the desired bonding or plating action.
In view of the foregoing, it is the broad object of the present invention to provide improved apparatus for and methods of bonding and plating using an exploding foil. Additional specific objects of the invention are to minimize the formation of striations, thus increasing the uniformity of a bonded or plated area, and to maximize the efficiency of the explosion, thereby reducing undesirable heat and blast effects and energy consumption.